The present invention relates to a process for oxygen bleaching fibrous cellulose material.
In an article entitled "Oxygen bleaching kinetics at ultra-low consistency", Tappi Journal, December, 1987, by C.L. Hsu and Jeffrey S. Hsieh, it is described experiments with oxygen bleaching to study the influence of temperature on the viscosity. The results show that a high temperature in the initial stage of the oxygen bleaching has a negative effect on the viscosity.
However, in order to reach the normal range of kappa number in oxygen bleaching it is not possible to use a reaction temperature which is too low. For this purpose temperatures of about 100.degree. C. are necessary, or special measures must be taken. Temperature control has thus been proposed to be carried out in oxygen bleaching pulp of medium consistency, see the article entitled "Improvement of medium consistency oxygen bleaching through temperature control" by C.C. Courchene and V.L. Magnotta, 1984, "Oxygen Delignification", pages 11 to 15. The oxygen delignification was performed in a horizontal tube reactor of laboratory size, to which steam was supplied at several points along the reactor in order to create two or more zones in which different temperatures could be maintained, the first zone having the lowest temperature. The experiments indicated that the temperature control with a low initial temperature in the horizontal tube reactor resulted in an improved yield and improved viscosity with the same retention time. However, the temperature control described cannot be applied with any great success on an industrial scale because of the difficulty in achieving an exact temperature limit between two temperature zones. This is partly due to the difficulty of efficiently and quickly mixing steam into the pulp in a uniform manner and also to the fact that the horizontal tube reactor, which contains a screw for feeding the pulp, has an upper space which is not filled by pulp but will instead contain a steam phase which disturbs the temperature control and extends along the entire length of the horizontal tube reactor.